1. Field of the Invention
The present invention relates to a distance measuring apparatus for use in a photographic or electric camera and the like, and more particularly to a distance measuring apparatus for measuring the distance of an object while applying pulsating light having a predetermined period to the object.
2. Description of the Prior Art
Distance measuring apparatus are mounted nowadays on still cameras and video cameras. Among such apparatus, there is known a light projecting type distance measuring apparatus whereby light is directed to an object, light reflected from the object is received by a light receiving area of the camera, and the position of the reflected light incident upon the light receiving area is used to measure the distance to the object. This measuring method is advantageous in that even if the brightness of an object is low, the measurement can be performed correctly.
FIG. 4 shows an example of a conventional light projecting type distance measuring apparatus. A light projecting unit 10 is comprised by an LED 11 for emitting near-infrared rays and a light projector lens 12. Upon depressing a shutter release button by half the full stroke, the light projecting unit 10 projects a spot of light or a line of light upon an object 14 to be photographed. A first light receiving unit 16 is comprised by a light reception element 17, a mask 18 coupled to the light reception element at the front surface thereof, an IR filter 19 for passing only near-infrared rays, and a light reception lens 20. A second light receiving unit 21 is constructed of a light reception element 22, an IR filter 23, and a light reception lens 24.
The first and second light receiving units 16 and 21 are spaced by predetermined distances from the light projecting unit 10 so that the position of reflected light incident thereon varies with the object distance. Since the mask 18 with a light shielding area 18a of a sawtooth shape as shown in FIG. 5 is coupled to the light reception element 17 at the front surface thereof, a photoelectrically converted signal representative of the strength and incident position of reflected light is outputted from the light reception element 17. Since the second light receiving unit 21 uses no mask, a photoelectrically converted signal representative of only the brightness of the reflected light is obtained from the light reception element 22.
The photoelectrically converted signals are inputted via respective amplifiers 25 and 26 to an operation section 27. The operation section 27 calculates a signal ratio between the two photoelectrically converted signals and outputs a lens set position signal which corresponds to the signal ratio and depends only on the object distance. Upon depressing further the release button, the picture taking lens is moved to the position corresponding to the lens set position signal and thereafter, the shutter is actuated to expose a photographic film.
Near-infrared rays contained in ambient light are also incident on the first and second light receiving units 16 and 21. Therefore, the photoelectrically converted signals obtained by the light reception elements 17 and 22 contain the components of ambient light which decrease the precision of measurement. This problem can be solved based on the fact that most of the components of ambient light are DC components. Accordingly, a predetermined number of light pulses are generated by the light projecting unit 10 at constant intervals, and only the AC component of the photoelectrically converted signals from the light receiving units is derived. To this end, it is possible to provide a system wherein chopper amplifiers and an integrator are used in place of the above-described amplifiers 25 and 26, the DC component contained in the photoelectrically converted signals from the light receiving units 16 and 21 is removed, and thereafter the remaining AC component is integrated to detect a measured distance signal based on the integrated value.
The above distance measuring apparatus uses a trigonometric distance measurement method, wherein the incident light reflected from an object is detected based on the amplitude of a photoelectrically converted signal from the light receiving unit. Therefore, to measure a distance more precisely, small photoelectrically converted signals from the first and second light receiving units 16 and 21 are amplified to the extent that noises are not superposed thereon, whereas too great photoelectrically converted signals are applied to the amplifiers with reduced gain factors so as not to saturate the amplifiers and the integrator. To do this, in a video camera provided with distance measuring apparatus of the above type, the gain factor of the amplifiers is arranged to be changed in accordance with the output levels of the photoelectrically converted signals obtained during the previous distance measurement operation.
The above-described method of changing the gain factor is effective for the case wherein the same object is continuously photographed, as with a video camera; but it cannot be used for the case wherein the object frequently changes, as with a still camera. In particular, if a still camera is used, a distance measurement must be performed every time the object is changed, without referring to the results of the previous distance measurement operation. In addition, the distance measurement must be made quickly. Thus, the conventional method applicable to video cameras is not practical for still cameras.